An American has been sentenced to 35 years in prison for his role in helping Pakistani terrorists plan the deadly 2008 Mumbai attacks.

David Coleman Headley, 52, had pleaded guilty to carrying out spying missions on downtown Mumbai for the Pakistani terrorist organisation Lashkar-e-Taiba (LeT) that was behind the attacks.

He was sentenced on 12 counts in a Chicago jail but had struck a plea deal with prosecutors to avoid the death penalty or extradition to India.

Headley, a Pakistani American from Chicago, visited India five times between 2006 and 2008, and created maps and took video footage and GPS coordinates of several of the attack sites, including the Taj Mahal hotel, Oberoi hotel and Nariman House Jewish centre.

His surveillance work provided vital information for the 10 LeT terrorists and their handlers, who launched the attack on November 26, 2008, holding the city under siege for three days and leaving 166 people dead, including British millionaire Andreas Liveras.

"I don't have any faith in Mr Headley when he says he's a changed person and believes in the American way of life," said Judge Harry Leinenweber. "The sentence I impose, I'm hopeful it will keep Mr Headley under lock and key for the rest of his natural life."

He added that Headley "deserved" the death penalty, but that he opted for the 35-year sentence after a motion by the government.

But US attorney Patrick Fitzgerald had urged leniency, telling the judge that Headley's decision to turn informant had "saved lives".

In a plot that reads like a spy thriller, Headley spent two years casing out Mumbai, even taking boat tours around the city's harbour to find landing sites for the attackers and befriending Bollywood stars as part of his cover.

Prosecutors described it as a supporting but "essential" role in the terrorist attack. He had begun plotting a second India attack during a March 2009 surveillance trip, but evidence from the only surviving gunman, Ajmal Amir Kasab, helped lead to his arrest later that year.

The Washington-born son of a former Pakistani diplomat and American woman, Headley's Western appearance and US passport helped him slip under the radar for much of the seven years he spent working with militant groups.

He was born Daood Gilani but changed his name to David Coleman Headley in 2006 "to present himself in India as an American who was neither Muslim nor Pakistani", prosecutors said.